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I/212 Proper motions in NGC 3680 (Kozhurina-Platais+, 1995)
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Catalogue of Proper Motions in the region of NGC 3680.
Kozhurina-Platais V., Girard T.M., Platais I., van Altena W.F.,
Ianna P.A., Cannon R.D.
<Bull. Inf. CDS 46, 9 (1995)>
=1995BICDS..46....9K
A proper motion study of the open cluster NGC 3680.
Ozhurina-Platais V., Girard M., Platais I., van Altena W.F.
<Astron. J. 109, 672 (1995)>
=1995AJ....109..672K
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ADC_Keywords: Clusters, open ; Positional data ; Proper motions ;
Magnitudes, photographic ;
Description (Abstract):
A catalogue of positions, proper motions and photographic BV
magnitudes for 2711 stars in and around the open cluster NGC 3680
is presented. The Catalogue is complete down to B=17.5 mag in a
rectangular, one square degree area centered on the cluster. The
limiting magnitude of the Catalogue is B=18.4 mag. The mean
standard error of the proper motions for well-measured stars is
+/-0.35 mas/yr. The photographic B and V magnitudes are accurate
to about +/-0.1 mag. The mean error of the relative positions for
all 2711 is +/-0.025 arcsec at the mean epoch of the plate
material, 1977.8. For each star, three different cluster
membership estimators are presented. For convenience,
cross-identifications to other studies are provided.
Introduction:
The open cluster NGC 3680 (C1123-429; l=286.8 deg, b=16.9 deg)
fits the same age group as the well-studied cluster M67, although
it is not a twin of that cluster. For instance, NGC 3680 contains
a much smaller number of cluster members. Most likely it belongs
to the group of so-called poorly-populated open clusters which has
been studied by Platais (1994). Unfortunately, the lack of an
astrometric study before now, impeded reaching definite
conclusions as to the cluster's basic parameters and the
luminosity function. Recently Kozhurina-Platais et. al (1995) have
completed a comprehensive proper motion study of NGC 3680. Here we
present an extended version of the proper motion catalogue for all
2711 stars measured in that study. The Catalogue should prove
valuable for further studies of the cluster properties and also
can be useful for galactic structure studies toward the galactic
anti-rotation direction as demonstrated by Mendez et al. (1993).
Data description:
A detailed description of the astrometric and photometric data is
available in Kozhurina-Platais et al. (1995). Only a brief summary
of the reductions and attained accuracies is provided here. The
astrometric reductions were based on twelve plates taken with the
Yale-Columbia 26-in refractor (plate scale 18.85 "/mm). The
earliest plate was taken in 1947, the latest - 1992. The largest
plates cover 67' (E-W) by 52' (N-S) area. All of the plates were
digitized with the Yale PDS microdensitometer equipped with laser
encoders. Image centroids were obtained by fitting a
two-dimensional bivariate Gaussian to the image density profiles.
Proper motions were calculated using the basic principles of an
iterative central-plate overlap technique supplemented with a very
careful account of the magnitude equation and the magnitude/color
corrections. Relative proper motions were calculated for 2711
stars, measured on at least three plates which spanned a minimum
of ten years epoch difference. The standard error of the proper
motion for bright (V<14.0 mag) stars, measured on all twelve
plates, is +/-0.35 mas/yr. The Catalogue of proper motions is
complete down to B=17.5 mag, while the limiting magnitude is
B=18.4 mag.
The five best 26-in telescope plates were used to obtain the B
magnitudes. V magnitudes were derived primarily from two visual
Yale Southern Observatory 51-cm astrograph plates. The
instrumental magnitudes were transformed into the standard BV
magnitudes using photographic BV-photometry by Anthony-Twarog et
al. (1991). The formal photometric error for the whole Catalogue
is about +/-0.1 mag in both B and V magnitude. Our BV photometry
should be used with caution in a detailed analysis of the
cluster's color-magnitude diagram since its primary purpose was
for application of magnitude and color corrections made in the
astrometric reductions. We are aware of several stars with totally
erroneous V-photometry (for instance, Nos. 376, 2696) caused by
the difficulty in separating blended images on the 51-cm
astrograph visual plates.
In addition to the proper motion, we also present each star's mean
position. The mean error of the relative positions for all 2711
stars is +/-0.45 micron or, in corresponding angular units,
+/-0.025 arcsec, and for the 524 stars brighter than V=14.5 mag,
the internal error is +/-0.18 micron (+/-0.010 arcsec) at the mean
epoch of the plate material, 1977.8. Note that we had no control
over possible magnitude equation in the positions. The mean
positions were updated to epoch=2000.0 and then transformed into
the HST Guide Star Catalog system of celestial coordinates using
219 reference stars. The unit weight mean error of this
transformation is +/-0.18 arcsec, which is dominated by the errors
in the reference frame.
Proper motion errors:
In the Catalogue the single-coordinate proper-motion formal error
is estimated from the scatter about the best-fit line for each
star separately. Due to the small number of plates involved in the
solution, this formal error may not always be a statistically
reliable estimator of the true error. Here we would like to
caution that the proper motion uncertainties given in
Kozhurina-Platais et al. (1995) for the probable cluster members
are calculated by median smoothing the proper-motion error
distribution as a function of B magnitude. This procedure,
however, was not applied to the stars with proper-motion errors
exceeding three times the median-smoothed error estimate. The
proper-motion membership probability calculation benefited
significantly from the use of these median-smoothed proper-motion
error estimates. Statistical properties of the formal errors,
though, should be studied only from the error values provided in
this Catalogue.
Membership Probabilities:
The probability of a given star being a cluster member is defined
as the ratio of the frequency of cluster stars to all stars at the
point within the proper motion distribution corresponding to that
star's proper motion. To derive the probabilities, the standard
technique (Sanders, 1971) of modeling the proper-motion
distributions has been widely used. Following this tradition, we
applied the modified Sanders method to the proper-motion data and
derived the probabilities P_µ. Our best fit to the proper-motion
distributions yields only 74 cluster members. With such a small
fraction of cluster members, the associated membership
probabilities may be sensitive to the shape of cluster's
luminosity function and the spatial distribution. The latter can
be rigorously taken into account by including an empirical spatial
surface density distribution into the membership probability. The
joint spatial/proper motion membership probabilities P_µ,r
provide more realistic probability estimates but can be distorted
by a varying cluster-to-field luminosity function ratio over the
entire magnitude range. Therefore, we also provide a so-called
local-sample membership estimator, P_s, which should not be biased
by possible differences in the shape of the luminosity functions.
Note that these probabilities are calculated only for stars
measured on seven or more plates.
For practical cluster member selection purposes, we recommend use
of the P_µ,r probabilities with the caution not to use them to
derive information regarding the cluster's spatial structure.
A full discussion regarding the various membership probability
estimates can be found in Kozhurina-Platais et al. (1995).